1. Field of the Invention
This invention relates to a system for adding a controlled amount of air to exhaust gases in the exhaust manifold of an internal combustion engine downstream of the combustion chamber for the purpose of cleaning the exhaust gases.
2. Description of the Prior Art
In this type of exhaust cleaning system, the flow rate of additional air to be to be fed to the engine exhaust manifold from an air pump is metered by a flow control valve in response to vacuum signals from an on-off solenoid value which is under on-off-duty control of a microcomputer operating on input signals indicative of variations in the partial oxygen pressure in the exhaust gases as detected by the oxygen sensing element, the engine r.p.m., the exhaust gas temperature, the throttle opening of the carburetor and the intake manifold vacuum. The cleaning system normally employs a vacuum control value for providing a vacuum source of a predetermined level by controlling the vacuum which is drawn from the engine intake manifold.
The controlled vacuum is fed to the on-off-duty solenoid valve thereby producing a vacuum signals for operating the flow control valve. Namely, the feed rate of additional air to the engine exhaust gases is solely controlled by the on-off-duty solenold valve operating in response to command signals from microcomputer which processes various operational variables according to predetermined arithmetic formulas. In this connection, there is a trend of increasingly relying on the microcomputer for various controls and calculations such as the instructions for economical operation of the engine and the calculation of fuel consumption, and it is presumed that the burden of the microcomputer will be increased in the future. Therefore, it is desirable to lessen the burden of the controls and calculations which are born by the microcomputer, substituting them as much as possible by mechanical or pneumatic operations.
Further, in the above-described conventional system employing a vacuum control valve which produces output vacuum of a predetermined level, the control responsive to the engine load is effected by feeding a variable to the microcomputer indicative of the engine level load and producing vacuum signals by the on-off control of the solenoid valve, resulting in a large consumption of additional air.